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This prevents a crash when Chrome shuts down immediately and 
AppShimHostManager is destructed before InitOnFileThread is run.
Note this means AppShimHostManager can stay around longer than
BrowserProcessPlatformPart during initialization.

BUG=242941

Review URL: https://chromiumcodereview.appspot.com/20065004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221580 0039d316-1c4b-4281-b951-d872f2087c98